1. Field of the Invention
The present invention generally relates to thermopile-based thermal sensors. More particularly, this invention relates to a monolithically-integrated infrared sensor in which a transducer and its sensing circuit are combined on a single silicon substrate in a manner that enhances and protects the transducer output signal and reduces noise.
2. Description of the Related Art
A thermopile comprises a series of connected thermocouples, each made up of dissimilar electrically-resistive materials such as semiconductors and metals, and converts thermal energy into an electric voltage by a mechanism known as the Seebeck effect. As an example, U.S. Pat. No. 5,982,014 describes a microfabricated differential temperature sensor comprising multiple stacked thermopiles. The general structure and operational aspects of thermopiles are well known and therefore will not be discussed in any detail here.
Infrared sensors that make use of thermopiles are also known, as evidenced by U.S. Pat. No. 5,059,543 to Wise et al., which describes a thermopile-based infrared sensor comprising a thermopile fabricated on a single silicon substrate. Cold junctions are located on a rim that supports and surrounds a diaphragm. The hot junctions of the thermopile are located at the center of the diaphragm, where exposure to infrared radiation occurs. A shortcoming of the sensor structure and process disclosed by Wise et al. is the manner in which the rim and diaphragm are defined. According to one embodiment, the rim is heavily doped for the purpose of serving as an etch stop during a wet chemical etch used to define the rim and diaphragm, thereby providing front-to-back alignment. The high dopant concentration and thermal treatment required for the rim to perform as an etch stop is incompatible with standard CMOS devices, necessitating that the sensor must be fabricated on a separate chip from its signal processing circuitry. This aspect of Wise et al. is disadvantageous because signal noise increases with increasing distance that a signal must travel to its processing circuitry. Voltages generated by the Seebeck effect are very small (in microvolts) and thus very difficult to detect with typical methods. While the voltage output of a thermopile can be increased with increasing numbers of thermocouples, the series resistance of the thermopile also increases. The resulting high impedance transducer output is particularly susceptible to the external noise that would be associated with the device of Wise et al.
U.S. Pat. No. 5,689,087 to Jack describes a thermopile-based radiation sensor that may include support circuitry integrated on the same substrate as the sensor. However, shortcomings or disadvantages of jack""s device include the requirement for using materials and process steps that are not standard in a CMOS high volume IC fabrication process and thus are not conducive to mass production processes. Finally, an article authored by M xc3xc ller et al., entitled A Thermoelectric Infrared Radiation Sensor with Monolithically Integrated Amplifier Stage and Temperature Sensor, Sensors and Actuators, A 54 (1996) 601-605, discloses a single thermopile infrared sensor that makes use of SIMOX (separation by implanted oxygen) technology to form an etch stop for etching. Mxc3xcller et al. (and the previously reported art) do not provide any on-chip calibration capability that enables calibration after packaging to allow for compensation variations that may occur in the packaging process.
The present invention is an integrated sensor comprising a thermopile transducer and signal processing circuit that are combined on a single semiconductor substrate, such that the transducer output signal is sampled in close proximity by the processing circuit. The transducer is adapted for sensing infrared radiation, and the sensor preferably includes features that promote absorption of thermal radiation within a portion of the sensor structure.
Generally, the sensor comprises a frame formed of a semiconductor material that is not heavily doped, and with which a diaphragm is supported for receiving thermal radiation. The diaphragm comprises multiple layers that include a first dielectric layer, a sensing layer containing at least a pair of interlaced thermopiles, a second dielectric layer, and a first metal layer defining metal conductors that electrically contact the thermopiles through openings in the second dielectric layer. Each thermopile comprises a sequence of thermocouples, each thermocouple comprising dissimilar electrically-resistive materials that define hot junctions located on the diaphragm and cold junctions located on the frame. Finally, the sensor includes signal processing circuitry on the frame and electrically interconnected with the thermopiles through the metal conductors defined by the first metal layer. The thermopiles are interlaced so that the output of a first of the thermopiles increases with increasing temperature difference between the hot and cold junctions thereof, and so that the output of a second of the thermopiles decreases with increasing temperature difference between the hot and cold junctions thereof. As a result, the transducer produces a differential signal output that converts a substantial portion of the noise into common mode noise that can be filtered out, thereby increasing the resolution of the sensor.
As described above, signal noise is minimized because the transducer and its signal processing circuitry are fabricated on the same chip, thereby minimizing the distance that the unamplified transducer signal must be transmitted. In particular, the close proximity between the transducer and the signal processing circuitry, together with the use of symmetry, are used to minimize capacitive and inductive coupling to off-chip sources of electric and magnetic fields that would be potential sources of extraneous signals. Fabrication of the sensor structure does not require high dopant concentrations or thermal treatments that are incompatible with standard CMOS devices, such that the signal processing circuitry can make use of CMOS and BiCMOS technology. The sensor also does not require the use of materials and process steps that are not conducive to mass production processes made possible with CMOS and micromachining technology.
Other objects and advantages of this invention will be better appreciated from the following detailed description.